Fluid coupling devices (“fan drives”) of the viscous shear type have been popular for many years for driving engine cooling fans, primarily because their use results in substantial saving of engine horsepower. The typical fluid coupling device operates in an engaged and relatively high speed condition only when cooling is needed. Further, it operates in a disengaged, relatively lower speed condition when little or no cooling is required.
Currently, viscous fluid fan clutches or fan drives are used in automotive engine cooling assemblies; and additional applications are constantly being developed. Fan speed of the fan clutches can be controlled for more efficient vehicle operation and better cooling through control of internal fan clutch torque transfer.
Viscous fluid fan clutches generally include power input members and power output members. The power input member of the clutch is driven by an engine or by an auxiliary power source. The power output member drives the fan to draw cooling air through heat exchange elements of a radiator, oil cooler, and air conditioner condenser. These clutches use a viscous fluid for transferring torque from the power input member to the output member. In the operation of these devices, the power input member often rotates at a higher speed than the output member. The difference in their speeds is called slip or slippage. Slip represents a power loss in the fan drive clutch and ways are constantly being sought for providing failsafe systems for slip situations occurring during extreme operation condition heat.
Viscous drives are always slipping to some degree causing them to be incapable of turning at fully engaged peak operating speeds or at higher speeds than originally designed. Since viscous drives are continuously slipping, they are continuously generating heat, unlike friction clutch assemblies. Viscous drives are further limited in that the more engine cooling needed the larger and more costly the viscous drive and cooling fan that is required. Thus, for increased engine cooling requirements viscous drives can become impractical in size and cost.
Due to increased engine cooling requirements, a current desire exists for a fan drive system capable of not only providing an increased amount of cooling over traditional fan drive systems but also having the associated advantages of a viscous drive, as stated above, without the associated disadvantages. It is also desirable that the fan drive system be practical and reasonable in size and cost so as to be approximately similar to, and preferably not to exceed that of, traditional fan drive systems. The present invention is directed to these ends.